Battery fuel gauges using FET segment control to increase low current measurement accuracy

Abstract

A circuit comprises a battery field-effect transistor (FET) coupled between a battery and an electronic system so that current from the battery flows through the battery FET to the electronic system. A replica FET couples the battery to a current sensing circuit. One of the battery FET and the replica FET comprises a plurality of parallel FET segments having separate control terminals. A logic block provides a plurality of switch control signals based on a system state input. A switching circuit receives the switch control signals and selectively couples the separate control terminals of the FET segments to one of a first reference voltage terminal or a second reference voltage terminal to independently turn each FET segment on or off. The current sensing circuit provides a voltage indicative of the current flowing through the replica FET.

What is claimed is: 
     
       1. A circuit comprising:
 a field-effect transistor (FET) coupled between a battery and an electronic system, the FET having a first terminal configured to be coupled to the battery and a second terminal configured to be coupled to the electronic system, wherein current from the battery flows through the FET between the battery and the electronic system, and wherein the FET comprises a plurality of FET segments coupled in parallel by coupling sources of each of the plurality of FET segments and coupling drains of each of the plurality of FET segments, and each of the plurality of FET segments having separate control terminals; 
 a logic block having an input to receive a system state input and having an output to provide a plurality of switch control signals based on the system state input; 
 a switching circuit having first terminals coupled to the plurality of switch control signals and having second terminals independently coupled to the separate control terminals of the plurality of FET segments and configured to selectively couple the separate control terminals of the plurality of FET segments to one of a first reference voltage terminal or a second reference voltage terminal to independently apply the first reference voltage or the second reference voltage to the control terminal of each FET to turn each FET segment on or off, wherein the switching circuit is configured to control a resistance in a current path between the battery and the electronic system; and 
 an analog to digital converter having a first input terminal coupled to the first terminal of the FET, having a second input terminal coupled to the second terminal of the FET and having an output to provide a digital value corresponding to a voltage difference across the first and the second terminals of the FET, the voltage difference across the first and the second terminals of the FET is configured to represent a direct measure of a current flowing in the current path between the battery and the electronic system. 
 
     
     
       2. The circuit of  claim 1  wherein the switching circuit comprises a plurality of switches, each switch of the plurality of switches having a control terminal coupled to one of the plurality of switch control signals and a first terminal of each of the plurality of switches coupled to a separate control terminal of a corresponding one of the FET segments, wherein each switch selectively couples one of the separate control terminals of one of the plurality of FET segments to one of the first reference voltage terminal or the second reference voltage terminal to independently turn said FET segment on or off. 
     
     
       3. The circuit of  claim 1  wherein a first number of FET segments turned on in a first system mode is less than a second number of FET segments turned on in a second system mode when the first system mode has a corresponding lower battery current than in a second system mode. 
     
     
       4. The circuit of  claim 1  wherein the system state input includes at least one of state of clock operations, state of voltage regulators, state of software applications, state of current draw, and system mode. 
     
     
       5. The circuit of  claim 1  wherein the analog to digital converter has a third terminal to receive the switch control signals to scale the digital value based on the switch control signals. 
     
     
       6. A method comprising:
 coupling a battery current between an electronic system and a battery through a field-effect transistor (FET) between the battery and the electronic system, wherein the FET comprises a plurality of FET segments coupled in parallel by coupling sources of each of the plurality of FET segments together and coupling drains of each of the plurality of FET segments together, and wherein each of the FET segments having separate control terminals; 
 receiving a system state input in a logic block and producing a plurality of switch control signals based on the system state input; 
 selectively activating a plurality of switches using the switch control signals, wherein the plurality of switches selectively couple the separate control terminals of each of the plurality of FET segments to one of a first reference voltage or a second reference voltage to independently apply the first reference voltage or the second reference voltage to the control terminal of each FET to turn each FET segment on or off, wherein the switch control signals are configured to control a resistance in a current path between the battery and the electronic system; and 
 converting a voltage difference across the coupled-together sources and the coupled-together drains of the plurality of the FET segments to a digital value, the voltage difference is configured to represent a direct measure of a current flowing in the current path between the battery and the electronic system. 
 
     
     
       7. The method of  claim 6  further comprising:
 turning on a first number of FET segments in a first system mode; and 
 turning on a second number of FET segments in a second system mode, 
 wherein the first number of FET segments is less than the second number of FET segments when the first system mode has a corresponding lower battery current than in a battery current in the second system mode. 
 
     
     
       8. The method of  claim 6  wherein the system state input includes at least one of state of clock operations, state of voltage regulators, state of software applications, state of current draw, and system mode.